A useful, model-independent parameterization of this form factor which only relies on QCD dispersion relations is called the BGL parameterization. With this parameterization [1]},
\(\begin{split}G(z)^2 & = \frac{4r}{(1+r)^2} f_+(z)^2 \\f_+(z) & = \frac{1}{P_+(z) \phi _+(z)} \sum ^N_{n=o} a_{+,n} z^n \\\phi _+(z) & = 1.1213 (1+z)^2 (1-z)^{1/2} [(1+r)(1-z) + 2 \sqrt{r} (1+z)]^{-5}.\end{split}\) 
